Determine/Find any usage of a bone in a rig...how?
The outliner offers the best overview of your scene data that is available.

You can search the contents, one drawback is that the list stops at matches. That means you can search for bones that have constraints, but you won't see the constraints listed. The other thing is that it shows and searches by the name of items, you can rename an IK constraint to be "mydoohickey" and searching for "IK" will then not find it.



Searching for ik can show bones that have an IK constraint. Note that bone constraints only show when the rig is on pose mode.



To find bones used in an action, you probably want to look at the graph editor where you can search for bone names to see if they have any animation data.



For listing other info, we are left to using python scripts. For example to list bones that have an IK constraint -

import bpy

for b in bpy.context.object.pose.bones:
    for c in b.constraints:
        if c.type == 'IK':
            print(f'{b.name} has an IK constraint')